dark horse

beat a dead horse

To persist or continue far beyond any purpose, interest or reason.

charley horse

A muscle cramp, usually a strong one, in the thigh, leg, arm, or core.

close the stable door after the horse has bolted

To attempt to prevent a problem that has already occurred; to design and implement a preventive mechanism that is too late (after the first failure instead of before).

dark cloud

A situation or future event that causes worry, problems or unhappiness, likened to such a cloud overhead in the sky.

enough to choke a horse

An excessive quantity.

flog a dead horse

To attempt to get more out of something that cannot give more; to attempt to arouse fresh interest in something that is either hopeless or already settled.

get back on the horse

To return to an activity after a period of abstaining from the activity.

get back on the horse that bucked one

To return to something that previously caused harm.